An important aspect of many plant improvement programs is the availability of cytoplasmic male sterility (CMS) for use in producing hybrids, as disclosed in U.S. Pat. No. 4,517,763, the contents of which are hereby incorporated by reference. Since CMS in plants is the result of interaction between heritable nuclear and cytoplasmic factors, manipulation of those factors by sexual hybridization has been the strategy of choice when the availability of a male sterile line is deemed essential, for example, in producing F1 hybrid seed on a commercial scale. But sexual manipulation of CMS is limited by the fact that cytoplasmic elements, including the genetic determinants encoding CMS, are typically inherited from the maternal (seed-bearing) plant only, except in a few nonagronomic species.
By the same token, it is generally believed that the contribution of the male (pollen-producing) parent in agronomic species, including those of the genus Brassica, is restricted to nuclear genes. In other words, the solely maternal inheritance of cytoplasm has been viewed as a barrier to the combining of the respective cytoplasmic traits, such as CMS and triazine resistance, of each parent, except by the use of protoplast fusion and regeneration techniques to create somatic hybrids. But these latter techniques are not widely applicable across the range of agronomic species. In addition, they present problems of relative complexity and expense not usually encountered in conventional hybridization programs.